The present invention relates to an overvoltage protection circuit for a power semiconductor and a method for protecting a power semiconductor from over-voltages.
Power switches on the basis of semiconductor switches are known. Because such power semiconductor switches do not have any moving mechanical parts, they ensure a wear-free operation with very short switching times over a long period of time. Bipolar transistors with an insulated gate (IGBT) or metal-oxide-semiconductor field-effect transistors (MOSFET) are, for example, used as such power semiconductor switches. In order to protect these transistors, the power semiconductor switches are additionally equipped with a free-wheeling diode. Because the dielectric strength of such power semiconductor switches is limited, additional circuit means have to be in place in order to ensure that the voltage between input and output of the power semiconductor switch is always less than a so-called breakdown voltage. If the voltage between the terminals of the power semiconductor switch rises above said breakdown voltage, there is then the danger of the power semiconductor switch being destroyed. This destruction of said power semiconductor switch is irreversible.
So-called Zener diodes or also suppression diodes are therefore used in order to protect the power semiconductor switch from such dangerous over-voltages. Should, for example, an overvoltage spike occur due to an interrupted current flow in a load inductor, the corresponding power semiconductor switch is actively activated if the breakdown voltage of the Zener or suppression diode is exceeded. In so doing, a renewed current flow through the power semiconductor switch is possible, whereby the voltage spike is limited to a safe value. In addition, a controller is often situated in the circuit which limits a voltage at the output via an intervention into the control voltage of the power semiconductor switch. Such a controller has, however, low dynamics so that short overvoltage spikes cannot be compensated by the same, whereby the use of the previously described Zener or suppression diodes becomes necessary.
The German patent application DE 10 2010 008 815 A1 discloses a device for protecting a semiconductor from over-voltages, wherein the device is only then moved into an active state if the semiconductor switch is turned off. The overvoltage protection is implemented by means of protective circuitry comprising diodes.
A great deal of importance is therefore placed on protecting power semiconductor switches. Hence, there is a need for a reliable overvoltage protection for power semiconductors. There is particularly a need for an overvoltage protection for power semiconductors which is not very dependent on temperature. In addition, there is also a need for an overvoltage protection of power semiconductor switches which has a low tolerance requirement and a high resistance to ageing.